Dielectric ceramic compositions

ABSTRACT

A dielectric ceramic composition consisting essentially of: 
     
         Pb(M.sub.1/3 Nb.sub.2/3).sub.x (M&#39;.sub.a Nb.sub.1-a).sub.y (Fe.sub.2/3 
    
      W 1/3 ) z  O 3 , 
     wherein M represents Ni or Mg and x+y+z=1, and 
     when M is Ni, M&#39;=Fe, a=1/2, 0.01≦x≦0.40, 0.45≦y≦0.80 and 0.05≦z≦0.50; and 
     when M is Mg, M&#39;=Zn, a=1/3, 0.01≦x≦0.70, 0.15≦y≦0.45 and 0.05≦z≦0.60. 
     The ceramic composition allows low-temperature sintering and exhibits high dielectric constant.

BACKGROUND OF THE INVENTION

1. Field of the Invention:

The present invention relates to dielectric ceramic compositions allowing low-temperature sintering, and exhibiting high dielectric constant, and being suitable for use in multilayer ceramic capacitors.

2. Description of the Prior Art:

As promising ceramic compositions for this application, ceramic compositions essentially consisting of BaTiO₃ have been widely used as a materials of high dielectric constant. However, the BaTiO₃ system ceramics must be sintered at a very high temperature in the range of 1300° C. to 1400° C., so that where they are used as a dielectric material of multilayer ceramic capacitors, expensive metals such as platinum or palladium which can stand such high sintering temperature should be used as internal electrodes of the capacitors. Therefore there has still been a demand for dielectric ceramic materials which can be sintered at a temperature as low as below 1000° C. for enabling the use of relatively cheap metal such as silver for the internal electrodes.

U.S. Pat. No. 4,078,938 has described binary system ceramic compositions of PbFe_(2/3) W_(1/3) O₃ --PbFe₁₇₈ Nb_(1/2) O₃ which can be sintered at a relatively low temperature, and which exhibit high dielectric constant. However, the ceramics have low specific resistance.

U.S. Pat. No. 4,236,928 has described binary system ceramic compositions of Pb(Fe_(2/3) W_(1/3))O₃ --Pb(Zn_(1/3) Nb_(2/3))O₃ which can be sintered at a temperature below 1000° C. However, the ceramics have relatively low dielectric constant.

SUMMARY OF THE INVENTION

It is one object of the present invention to provide excellent dielectric ceramic compositions which have low sintering temperature, high dielectric constant and high specific resistivity.

Another object of the present invention is to provide dielectric ceramic compositions which have low sintering temperature along with high dielectric constant and low temperature coefficient of the dielectric constant.

In accomplishing these objects, a ceramic composition of the present invention has a dielectric ceramic composition consisting essentially of a material represented by the formula

    Pb(M.sub.1/3 Nb.sub.2/3).sub.x (M'.sub.a Nb.sub.1-a).sub.y (Fe.sub.2/3 W.sub.1/3).sub.z O.sub.3,

wherein

M represents one element selected from the group consisting of Ni and Mg, M'=Fe, a=1/2, 0.01≦x≦0.40, 0.45≦y≦0.80 and 0.05≦z≦0.50 when Ni is selected for M, M'=Zn, a=1/3, 0.01≦x≦0.70, 0.15≦y≦0.45 and 0.05≦z≦0.60 when Mg is selected for M, and x+y+z=1.

These novel compositions of the present invention can be sintered at a temperature as low as 1000° C. or less, and have a high dielectric constant, a high specific resistivity and a low temperature coefficient of the dielectric constant.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is based on the discovery that within certain particular compositional ranges of these systems Pb(Ni_(1/3) Nb_(2/3))O₃ --Pb(Fe_(1/2) Nb_(1/2))O₃ --Pb(Fe_(2/3) W_(1/3))O₃ and Pb(Mg_(1/3) Nb_(2/3))O₃ --Pb(Zn_(1/3) Nb_(2/3))O₃ --Pb(Fe_(2/3) W_(1/3))O₃ the specimens exhibit very high dielectric constant along with low sintering temperature.

The compositions described herein may be prepared in accordance with various well-known ceramic procedures.

EXAMPLE 1

The starting materials, viz., lead oxide (PbO), nickel oxide (NiO), ferric oxide (Fe₂ O₃), niobium oxide (Nb₂ O₅) and tungsten oxide (WO₃), all of relatively pure grade, were intimately mixed in a ball mill with distilled water. Thereafter the mixtures were dried and then calcined at 750° C. for 2 hours. Thus obtained materials were wet-ground in a ball mill, dried, mixed with polyvinyl alcohol as a binder solution, and then pressed into columns of about 13 mm in diameter and about 8 mm in length at a pressure of 700 kg/cm². After burning out binder at about 700° C., the pressed columns which were put into magnesia crucible were sintered at a temperature of 880° C. to 1040° C. for 2 hours. The sintered bodies were cut into discs of about 1 mm in thickness, and then were attached Cr--Au electrodes on the both surfaces of the discs by a method of vacuum evaporation. Various properties of the ceramic discs thus obtained are shown in Table 1. The dielectric constant (ε_(r)) and the dielectric loss (D) were measured at a frequency of 1 kHz and a voltage of 1 V under room temperature. The temperature coefficient of a dielectric constant was obtained by measuring a dielectric constant at a temperature range of -25° C. to 85° C. and calculated with reference to the dielectric constant at 20° C. The specific electrical resistivity was measured at room temperature by applying a D.C. voltage of 1 kV.

                                      TABLE 1                                      __________________________________________________________________________                 Sintering                                                                      temper-       Change of                                                                               Specific                                    Compositions                                                                               ature    D    ε.sub.r (%)                                                                     resistivity                                 No.                                                                               x  y  z  (°C.)                                                                        ε.sub.r                                                                    (× 10.sup.-4)                                                                 -25° C.                                                                      85° C.                                   cm)                                (Ω                                    __________________________________________________________________________      1*                                                                               0  0.50                                                                              0.50                                                                              940  8740                                                                                41   72  -75 1.5 × 10.sup.7                        2  0.01                                                                              0.59                                                                              0.40                                                                              920  18690                                                                              245  -74  -69 6.1 × 10.sup.8                         3*                                                                               0.05                                                                              0.85                                                                              0.10                                                                              920  6130                                                                               576  -62   44 1.6 × 10.sup.9                        4  0.05                                                                              0.45                                                                              0.50                                                                              900  8570                                                                                80   36  -59 2.3 × 10.sup.9                         5*                                                                               0.05                                                                              0.40                                                                              0.55                                                                              880  5710                                                                                72   55  -53 7.5 × 10.sup.8                        6  0.10                                                                              0.80                                                                              0.10                                                                              900  8260                                                                               483  -58   27 3.2 × 10.sup.11                       7  0.10                                                                              0.60                                                                              0.30                                                                              880  22620                                                                              260  -68  -76 7.1 × 10.sup.11                       8  0.10                                                                              0.50                                                                              0.40                                                                              900  13450                                                                               67  -15  -71 6.5 × 10.sup.11                       9  0.20                                                                              0.60                                                                              0.20                                                                              940  23410                                                                              136  -71  -75 5.0 × 10.sup.11                       10 0.30                                                                              0.60                                                                              0.10                                                                              960  15950                                                                              293  -43  -60 4.3 × 10.sup.11                       11 0.30                                                                              0.50                                                                              0.20                                                                              960  11290                                                                              442  -27  -53 8.9 × 10.sup.10                       12 0.40                                                                              0.55                                                                              0.05                                                                              980  8230                                                                               124   16  -47 1.7 × 10.sup.11                       13*                                                                               0.45                                                                              0.55                                                                              0  1020 7400                                                                               526  -56  -29 5.2 × 10.sup.10                       14*                                                                               0.50                                                                              0.40                                                                              0.10                                                                              1040 4280                                                                               232   32  -64 7.4 × 10.sup.10                       __________________________________________________________________________      Compositions of the Nos. with an asterisk are outside the scope of the         present invention.                                                             Compositions: Pb(Ni.sub.1/3 Nb.sub.2/3).sub.x (Fe.sub.1/2                      Nb.sub.1/2).sub.y (Fe.sub.2/3 W.sub.1/3).sub.z O.sub.3.                  

From Table 1 it is obvious that ceramic compositions Nos. 2, 4, and 6 to 12 within the scope of the present invention provide high dielectric constant (ε_(r) =8230--23410) and low dielectric loss (D<500×10⁻⁴), and can be sintered at a temperature below 1000° C., and further provide high specific resistivity and low temperature coefficient of dielectric constant.

In the ceramic compositions represented by the formula Pb(Ni_(1/3) Nb_(2/3))_(x) (Fe_(1/2) Nb_(1/2))_(y) (Fe_(2/3) W_(1/3))_(z) O₃ wherein x<0.01, the specific resistivity of the ceramics is low. In the compositions of x>0.40 , the ceramics cannot be sintered at a temperature below 1000° C. And the ceramic compositions of y<0.45, y>0.80, z<0.05 and/or z>0.50 provide relatively small dielectric constant. Therefore such compositions are not suited for use as a capacitors.

EXAMPLE 2

The starting materials, viz., lead oxide (PbO), nickel oxide (NiO), ferric oxide (Fe₂ O₃), niobium oxide (Nb₂ O₅), tungsten oxide (WO₃), manganese dioxide (MnO₂), cromium oxide (Cr₂ O₃), cobalt oxide (CoO) and lithium carbonate (Li₂ CO₃), all of relatively pure grade, were intimately mixed in a ball mill with distilled water. Thereafter the mixtures were dried and then calcined at 750° C. for 2 hours. Thus obtained materials were wet-ground in a ball mill, dried, mixed with polyvinyl alcohol as a binder solution, and then pressed into columns of about 13 mm in diameter and about 8 mm in length at a pressure of 700 kg/cm². After burning out binder at about 700° C., the pressed columns which were put into magnesia crucible were sintered at a temperature of 840° to 980° C. for 2 hours. The sintered bodies were cut into discs of about 1 mm in thickness, and then were attached Cr--Au electrodes on the both surfaces of the discs by a method of vacuum evaporation. ε_(r), D and specific electrical resistivity of the ceramic discs are shown in Table 2. ε_(r) and D were measured at a frequency of 1 kHz and a voltage of 1 V under room temperature. The specific resistivity was measured at room temperature by applying a D.C. voltage of 1 kV.

                                      TABLE 2                                      __________________________________________________________________________                              Sintering                                             Compositions             temper-       Specific                                            Additives    ature    D    resistivity                             No.                                                                               x  y  z  (wt %)       (°C.)                                                                        ε.sub.r                                                                    (× 10.sup.-4)                          cm)                                    (Ω                                __________________________________________________________________________     15 0.01                                                                              0.59                                                                              0.40                                                                              --           920  18690                                                                              245  6.1 × 10.sup.8                    16 "  "  "  0.2 MnO.sub.2                                                                               920  17920                                                                              153  7.5 × 10.sup.10                   17 "  "  "  0.2 Cr.sub.2 O.sub.3                                                                        920  16870                                                                              107  4.1 × 10.sup.10                   18 "  "  "  0.2 CoO      920  18150                                                                              124  3.9 × 10.sup.10                   19 "  "  "  0.2 Li.sub.2 O                                                                              920  19430                                                                               82  5.4 × 10.sup.10                   20 0.05                                                                              0.45                                                                              0.50                                                                              --           900   8570                                                                               80  2.3 × 10.sup.9                    21 "  "  "  0.01 MnO.sub.2                                                                              900   8810                                                                               72  4.3 × 10.sup.9                    22 "  "  "  0.01 Cr.sub.2 O.sub.3                                                                       900   8490                                                                               65  4.1 × 10.sup.9                    23 "  "  "  0.01 CoO     900   8530                                                                               68  5.7 × 10.sup.9                    24 "  "  "  0.01 Li.sub.2 O                                                                             900   9070                                                                               52  3.3 × 10.sup.9                    25 0.10                                                                              0.80                                                                              0.10                                                                              --           900   8260                                                                              483  3.2 × 10.sup.11                   26 "  "  "  0.1 MnO.sub.2                                                                               900   8200                                                                              195  6.1 × 10.sup.11                   27 "  "  "  0.1 Cr.sub.2 O.sub.3                                                                        900   8440                                                                              241  8.3 × 10.sup.11                   28 "  "  "  0.1 CoO      900   8610                                                                              307  5.0 × 10.sup.11                   29 "  "  "  0.1 Li.sub.2 O                                                                              900   8150                                                                              239  9.4 × 10.sup.11                   30 0.10                                                                              0.60                                                                              0.30                                                                              --           880  22620                                                                              260  7.1 × 10.sup.11                   31 "  "  "  0.5 MnO.sub.2                                                                               880  20590                                                                              207  2.5 × 10.sup.12                   32 "  "  "  1.5 MnO.sub.2                                                                               840  16200                                                                               98  2.1 ×  10.sup.12                  33 "  "  "  0.2 MnO.sub.2 + 0.2 CoO                                                                     900  21730                                                                              134  4.8 × 10.sup.12                   34 "  "  "  0.5 Li.sub.2 O                                                                              900  24100                                                                              123  9.8 × 10.sup.12                   35 "  "  "  1.5 Li.sub.2 O                                                                              900  17350                                                                              214  1.3 × 10.sup.12                   36 "  "  "  0.2 MnO.sub.2 + Li.sub.2 O                                                                  880  23470                                                                               72  8.3 × 10.sup.12                   37 0.20                                                                              0.60                                                                              0.20                                                                              --           940  23410                                                                              136  5.0 × 10.sup.11                   38 "  "  "  0.05 CoO     940  22160                                                                              108  7.2 × 10.sup.11                   39 "  "  "  0.2 CoO      920  21500                                                                               96  4.3 × 10.sup.12                   40 "  "  "  1.5 CoO      920  14020                                                                              115  8.1 × 10.sup.11                    41*                                                                              "  "  "  2.0 CoO      920   9380                                                                              324  2.3 × 10.sup.11                   42 "  "  "  0.2 Cr.sub.2 O.sub.3                                                                        940  24340                                                                              123  7.5 × 10.sup.12                   43 "  "  "  1.5 Cr.sub.2 O.sub.2                                                                        940  16600                                                                              101  5.9 × 10.sup.11                    44*                                                                              "  "  "  2.0 Cr.sub.2 O.sub.3                                                                        940  10130                                                                              157  9.4 × 10.sup.10                   45 "  "  "  0.2 CoO + 0.2 Cr.sub.2 O.sub.3                                                              940  21940                                                                               89  5.3 × 10.sup.12                   46 "  "  "  0.2 CoO + 0.1 Li.sub.2 O                                                                    900  18350                                                                              108  7.4 × 10.sup.12                   47 "  "  "  0.2 Cr.sub.2 O.sub.3 + 0.1 Li.sub.2 O                                                       900  19610                                                                               84  6.4 × 10.sup.12                   48 "  "  "  0.1 MnO.sub.2 + 0.1 CoO +                                                                   900  21090                                                                               98  5.7 × 10.sup.12                               0.1 Cr.sub.2 O.sub.3                                               49 "  "  "  0.1 MnO.sub.2 +0.1 CoO +                                                                    880  18720                                                                              110  2.0 × 10.sup.12                               0.1 Cr.sub.2 O.sub.3 + 0.1 Li.sub.2 O                              50 0.30                                                                              0.60                                                                              0.10                                                                              --           960  15950                                                                              293  4.3 × 10.sup. 11                  51 "  "  "  0.2 MnO.sub.2                                                                               960  13490                                                                              218  8.0 × 10.sup.12                    52*                                                                              "  "  "  2.0 MnO.sub.2                                                                               920   7410                                                                              472  2.1 × 10.sup.11                   53 "  "  "  0.2 Li.sub.2 O                                                                              940  17020                                                                              240  5.2 × 10.sup.12                    54*                                                                              "  "  "  2.0 Li.sub.2 O                                                                              880   6740                                                                              325  6.4 × 10.sup.10                   55 0.30                                                                              0.50                                                                              0.20                                                                              --           960  11290                                                                              442  8.9 × 10.sup.10                   56 "  "  "  0.1 MnO.sub.2                                                                               960  12030                                                                              279  2.7 × 10.sup.11                   57 "  "  "  0.1 MnO.sub.2 + 0.1 Cr.sub.2 O.sub.3                                                        960  10170                                                                              194  1.3 × 10.sup.12                   58 "  "  "  0.1 MnO.sub.2 + 0.1 Cr.sub.2 O.sub.3 +                                                      920  11400                                                                              213  2.8 × 10.sup.12                               0.1 Li.sub.2 O                                                     59 0.40                                                                              0.55                                                                              0.05                                                                              --           980   8230                                                                              124  1.7 × 10.sup.11                   60 "  "  "  0.1 MnO.sub.2                                                                               980   8610                                                                               62  2.9 × 10.sup.12                   61 "  "  "  0.1 Cr.sub.2 O.sub.3                                                                        980   8500                                                                               49  5.6 × 10.sup.11                   62 "  "  "  0.1 CoO      980   8140                                                                               70  3.1 × 10.sup.12                   63 "  "  "  0.1 Li.sub.2 O                                                                              980   8160                                                                               72  4.5 × 10.sup.11                   __________________________________________________________________________      Basic compositions: Pb(Ni.sub.1/3 Nb.sub.2/3).sub.x (Fe.sub.1/2                Nb.sub.1/2).sub.y (Fe.sub.2/3 W.sub.1/3).sub.z O.sub.3.                        Compositions of the Nos. with an asterisk are outside the scope of the         present invention.                                                       

It is obvious from Table 2 that Pb(Ni_(1/3) Nb_(2/3))_(x) (Fe_(1/2) Nb_(1/2))_(y) (Fe_(2/3) W_(1/3))_(z) O₃ ceramics containing at least one element selected from the group consisting of Mn, Cr, Co and Li in a total quantity equivalent to from 0.01 to 1.5 weight percent of respective oxides (MnO₂, Cr₂ O₃, CoO and Li₂ O) exhibit high specific electrical resistivity as compared with that of compositions with no addition, exhibit low D as compared with that of the ceramics with no addition, and exhibit still high ε_(r) (8140-24100) along with low sintering temperature (840°-980° C.). In the compositions containing over 1.5 wt. % of MnO₂, Cr₂ O₃, CoO or Li₂ O as additive the specific electrical resistivity of the ceramics is low.

EXAMPLE 3

The starting materials, viz., lead oxide (PbO), magnesium oxide (MgO), zinc oxide (ZnO), ferric oxide (Fe₂ O₃), niobium oxide (Nb₂ O₅) and tungsten oxide (WO₃), all of relatively pure grade, were intimately mixed in a ball mill with distrilled water. Thereafter the mixture were dried and then calcined at about 750° C. for 2 hours. Thus obtained materials were wet-ground in a ball mill, dried, mixed with polyvinyl alcohol as a binder solution, and then pressed into columns of about 13 mm in diameter and about 8 mm in length at a pressure of 700 kg/cm². After burning out binder at about 700° C., the pressed columns which were put into magnesia crucible were sintered at a temperature of 780° to 1040° C. for 2 hours. The sintered bodies were cut into discs of about 1 mm in thickness, and then were attached Cr--Au electrodes on the both surfaces of the discs by a method of vacuum evaporation. ε_(r), D and change of ε_(r) with temperature of the ceramic discs are shown in Table 3. ε_(r) and D were measured at a frequency of 1 kHz and a voltage of 1 V under room temperature. The change of ε_(r) with temperature were measured at a temperature range of -25 to 85° C. and calculated with reference to the ε_(r) at 20° C.

                                      TABLE 3                                      __________________________________________________________________________                  Sintering                                                                      temper-         Change of ε.sub.r                         Compositions ature     D     (%)                                               No.                                                                               x  y   z  (°C.)                                                                         ε.sub.r                                                                    (× 10.sup.-4)                                                                  -25° C.                                                                      85° C.                                __________________________________________________________________________      64*                                                                              -- 0.40                                                                               0.60                                                                              920   5830                                                                                95    28  -43                                          65 0.01                                                                              0.40                                                                               0.59                                                                              900   6650                                                                               108    26  -39                                          66*                                                                               0.10                                                                              0.50                                                                               0.40                                                                              860   5240                                                                               281   -33  -25                                          67 0.10                                                                              0.45                                                                               0.45                                                                              820   6490                                                                               235    -8  -28                                          68 0.10                                                                              0.40                                                                               0.50                                                                              780   8120                                                                               223    16  -35                                          69 0.10                                                                              0.30                                                                               0.60                                                                              820   6380                                                                                67    27  -38                                           70*                                                                              0.10                                                                              0.25                                                                               0.65                                                                              840   4900                                                                                85    36  -52                                          71 0.20                                                                              0.40                                                                               0.40                                                                              820   8210                                                                                98    -9  -34                                          72 0.20                                                                              0.30                                                                               0.50                                                                              800   7300                                                                                57    22  -37                                          73 0.30                                                                              0.40                                                                               0.30                                                                              840   9670                                                                               112   -31  -34                                          74 0.40                                                                              0.40                                                                               0.20                                                                              820   11820                                                                              203   -33  -35                                          75 0.40                                                                              0.30                                                                               0.30                                                                              820   10530                                                                               64   -21  -40                                          76 0.40                                                                              0.15                                                                               0.45                                                                              860   6440                                                                                71    28  -35                                          77 0.50                                                                              0.30                                                                               0.20                                                                              900   8790                                                                               135   -16  -30                                          78 0.60                                                                              0.30                                                                               0.10                                                                              940   8980                                                                               183   -29  -31                                          79 0.70                                                                              0.25                                                                               0.05                                                                              960   8210                                                                               236   -34   12                                          80 0.70                                                                              0.15                                                                               0.15                                                                              940   6550                                                                               124   -15  -32                                           81*                                                                              0.70                                                                              0.10                                                                               0.20                                                                              1000  5670                                                                                98     9  -46                                           82*                                                                              0.73                                                                              0.25                                                                               0.02                                                                              1040  8420                                                                               269   -47    7                                          __________________________________________________________________________      Compositions of the Nos. with an asterisk are outside the scope of the         present invention.                                                             Compositions: Pb(Mg.sub.1/3 Nb.sub.2/3).sub.x (Zn.sub.1/3                      Nb.sub.2/3).sub.y (Fe.sub.2/3 W.sub.1/3).sub.z O.sub.3.                  

It is clear from Table 3 that ceramic compositions Nos. 65, 67, 68 and 71 to 80 within the scope of present invention provide high ε_(r) (6380-11820), low D (≦236×10⁻⁴) and low temperature coefficient of ε_(r) along with low sintering temperature (780°-960° C.).

The ceramic compositions represented by the formula Pb(Mg_(1/3) Nb_(2/3))_(x) (Zn_(1/3) Nb_(2/3))_(y) (Fe_(2/3) W_(1/3))_(z) O₃ wherein x<0.01, y<0.15, y>0.45 and/or z>0.60, provide relatively small ε_(r). In the compositions of x>0.70 and/or z<0.05, the ceramics cannot be sintered at a temperature below 1000° C.

As apparent from these Examples 1 to 3, the ternary ceramic compositions of the present invention have low sintering temperature below 1000° C., so that relatively cheap metal such as silver can be employed as internal electrodes of multilayer ceramic capacitors, and the durability of furnace materials for sintering use may be extended and electric power for sintering may be lowered. Moreover, the ceramic compositions according to the present invention exhibit high dielectric constant along with high specific electrical resistivity, low temperature coefficient of the dielectric constant and/or low dielectric loss. Therefore, the ceramic compositions are suitable for use of ceramic capacitors.

It will be evident that the starting materials to be used in the present invention are not limited to those used in the above examples. Other oxide or compounds which are easily decomposed at elevated temperature may be used in place of the starting materials of the above examples. 

What is claimed is:
 1. A ceramic composition having a dielectric constant of 6380 or higher, and a sintering temperature of 960° C. or lower and consisting essentially of

    Pb(Mg.sub.1/3 Nb.sub.2/3).sub.x (Zn.sub.1/3 Nb.sub.2/3).sub.y (Fe.sub.2/3 W.sub.1/3).sub.z O.sub.3

wherein 0.01 ≦x≦0.70, 0.15≦y≦0.45, 0.05≦z≦0.60 and x+y+z=1. 